drm/radeon/kms: fix dp_detect handling for DP bridge chips

The HPD pin is not reliable for detecting whether a monitor
is connected or not.  Skip HPD and just use DDC or load
detection.

Fixes phantom VGA connected bugs.

[Michel: fixes phantom VGA bugs on his llano system.]

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>